Who in the world told you a single month course would be adequate? Did you initially tell the gym you were applying for a DTV, did they give you paperwork?
Those are pretty much the documents needed. The 500k must be in the bank for 3 months. You will be called in for an interview and you will need to show your online banking as well as answer a few simple related questions about your application.
To qualify for the METV you must show a trip out of Thailand within the first 60 days and then a trip back to Thailand anytime after that. The METV is issued based on need so you have to show your travel plans otherwise you'll just get a single entry tourist Visa. No need to talk about the course you're taking or plan to take. With a well-timed border bounce you can get almost 9 months out of the METV Visa
It is totally irrelevant how long you intend to stay, or how many times you enter and exit. The application simply asks for proposed or intended dates and flight as well as length of stay, nothing you enter is binding.